CLIMATIC TYPES 928 



Where long rivers carry sediments from one climatic zone into another, 

 we have another variety of the mixed climatic type. There is no better 

 example of this than the Nile, although the Mississippi and many other 

 streams of great length have much in common with it. In some respects 

 the sediments of these rivers are mnch like those produced by annual 

 changes from wet to dry climate in the same area; but in others new 

 factors are introduced and the results vary correspondingly. 



INTERMEDIATE TYPES 



Thus far we have, of course, dealt only with the more distinct climatic 

 types, neglecting the fact that they all intergrade with one another 

 through an almost infinite variety of intermediate stages. A few are so 

 distinctive as to merit special mention, even in so brief a review as this. 

 Because of vast areas affected, I select two of the most important, namely, 

 the temperate subarid and the temperate suhhumid types of deposit. 



In regions like the Great Plains of the United States and the steppes 

 of southern Eussia and Siberia, where the rainfall is occasional rathei 

 than seasonal, and yet is sufficient for the development of a grassy turf, 

 though not for the growth of a forest cover, the streams and the wind vie 

 with each other for mastery in the formation of the sediments. The river 

 deposits usually present an averaging of the characteristics of the sub- 

 Arctic, the moist tropical and the desert types, being but imperfectly 

 either decayed, hydrated, or oxidized, and being associated with a small 

 amount of partly decayed vegetable matter. They therefore contain many 

 unaltered silicates, such as feldspar and mica, along with some kaolin and 

 iron oxides. Their characteristic colors are buff and brown, with black 

 very subordinate. There is but little peat, and on the other hand rarely 

 deposits of salts. Sun-cracks are abundantly developed, but the conditions 

 for their preservation are not quite so good as in the arid and seasonally 

 arid regions. The most characteristic sediment of this climatic type is 

 undoubtedly the loess. Most students of the subject now regard the loess 

 as being the dust carried from more arid regions out over the adjacent 

 prairie or grassy lands, where it is entrapped by the turf and held in con- 

 stantly increasing thickness. This eolian loess, in turn, is subject to re- 

 working by the streams, which then impress on it their appropriate modi- 

 fications and thereby convert it into "river loess." True eolian loess con- 

 sists largely of grains and splinters of quartz, undecayed silicates, and 

 even carbonates. The proportions of kaolin and iron oxides are relatively 

 low. Its obscure stratification, its pale buft" color, its dry-land fossils, 

 and its ability to stand in vertical cliff's are well known. A genetic rela- 

 tion to glaciation has been urged by certain geologists for some of our 



